In this fanfiction, it could be that Percy is chosen for a special task by the existing Olympians. They see his potential not just as a hero but as someone who could join their ranks. So he gets trained in Olympian magic and lore, which is very different from what he's used to. He has to learn how to control new powers and also deal with the politics among the Olympians.
One possible plot is that Percy discovers a new aspect of his heritage that links him directly to the Olympians in a way that no one expected. This newfound connection gives him the opportunity to ascend to Olympian status. But there are those who oppose his rise, either out of jealousy or fear of change. So Percy has to fight against these forces while also coming to terms with his own new identity as an almost - Olympian.
Well, perhaps Percy has to go through a series of trials. He might need to prove his worthiness to the Olympians. Maybe he has to battle some mythical creatures that are threatening the Olympian realm. And in the end, he gains the recognition and powers of an Olympian.
In this kind of fanfiction, one possible plot could be Percy having to deal with the politics among the Olympian gods. He might try to bring about changes, like making the gods more involved in mortal affairs in a positive way. For example, he could convince them to be more proactive in protecting the environment which is being damaged by humans. Also, Percy would probably have to face new enemies. Since he's now an Olympian, he could attract the jealousy of some minor gods or ancient monsters who see him as a threat to the established order.
The Titans might still be a threat. Even though they were defeated before, they could have some remnants or followers who don't want Percy to become an Olympian because it would strengthen the Olympian side even more.
Well, it could be about Percy facing a new and powerful threat that even the Olympians can't handle easily. Maybe he discovers a hidden 13th Olympian who has been locked away for centuries and now is causing chaos.
It could be an ancient deity that was banished long ago. Maybe a primordial god or goddess who was considered too powerful or too dangerous for the other Olympians. For example, it could be Erebus, the god of darkness.
The plot could start with Percy being somehow influenced by Kronos, maybe through a dream or a cursed object. Then he slowly starts to act in ways that are out of character for him. He might begin to gather followers who are also attracted to the power that Kronos represents. Eventually, he could lead an uprising against the gods, believing that he can create a new world order under Kronos's ideology.
The plot could involve Percy using his demigod skills in a samurai - like way. Maybe he discovers an ancient Japanese weapon that has some sort of connection to the Greek gods. He could start training with a Japanese mentor who is secretly a descendant of a minor god. And then he has to use his new samurai skills to fight off some mythical creatures that are attacking both the Greek and Japanese mythological worlds.
The Titans could be the main antagonists. They might oppose Percy's ascension to Olympian status because they see it as a threat to their own plans of overthrowing the Olympians.
In a Percy Jackson becomes a goddess fanfiction, the story could start with some sort of magical accident or a powerful spell that turns him into a goddess. As a goddess, Percy would have to adapt to a whole new way of life. He'd no longer be just a hero on quests but a divine being with a different set of rules. His powers would likely be enhanced, and he might gain abilities like shape - shifting or more powerful control over water. His journey would involve not only coming to terms with his new identity but also using his new powers to protect the world in a different way than he did as a demigod.
One possible plot could be that Percy, with his natural heroic instincts, discovers a hidden world of dragon riders. Maybe he stumbles upon a dragon egg during one of his quests. Since he has a connection with water and dragons are often associated with elements, he could find a way to bond with the newly hatched dragon. This would lead him to learn the ways of the dragon riders, facing challenges from those who oppose a half - blood like him becoming a rider.